Air carrier First Officer reported a NMAC during cruise when a small aircraft maneuvering in the area appeared suddenly out of the clouds within a few hundred feet of the airliner. Flight crew did not take evasive action.

Narrative

While in cruise at 6000; we got a traffic call from ATC for a VFR Aircraft Y at 6500 traveling the same direction. Later we're informed Aircraft Y is turning away from us. Within a minute Aircraft Y turns back towards us and starts descending. We get an amber TCAS traffic alert but no RA. He pops out of the clouds within a few hundred feet of us. We pass in front of Aircraft Y; missing a collision; albeit closely. Aircraft Y didn't appear to have a mode C transponder as he never showed with an altitude on our or ATCs display. Cause: The VFR Aircraft Y was maneuvering do remain clear of clouds and made turns and descents towards our aircraft prior to consulting ATC. This caused Aircraft Y to pop out within a few hundred feet of our aircraft. Solution: Had Aircraft Y had a mode C transponder with altitude; we would have a better idea of their proximity and likely received an RA. An ATC vector may have also helped; but wasn't a guaranteed solution given Aircraft Y's lack of timely position and maneuver reports.